In order to promote adhesion between rubber and ferrous metals it is known to employ a variety of metallic salts as coatings to the metal or as an ingredient in a rubber composition. Somewhat typical of the first type of art is U.S. Pat. No. 1,919,718 which discloses a rubber cement composition to be used between a layer of vulcanizable rubber and a metal surface. The cement is provided with the salt of an organic acid with cobalt, copper, manganese or lead. The composition of the cement would not be suitable as a rubber stock inasmuch as it has too low a content of carbon black and too high a content of zinc oxide. Moreover, it is carried in a solvent.
U.S. Pat. No. 2,912,355 is directed toward improving the adhesion between rubber and metal by the incorporation into a rubber composition of a calcined, partially oxidized metal salt of an aliphatic fatty acid compound, the metal being cobalt, copper, iron, lead, mercury, nickel or silver.
Finally, U.S. Pat. No. 3,905,947 is directed toward a method for improving adhesion between vulcanizable rubber and metal surfaces by incorporating an organo-nickel compound into the rubber and then vulcanizing the latter in contact with the metal surface. In no instance is an inorganic compound employed or suggested.
While others have sought to enhance adhesion between rubber compositions and metals by employing various combinations of cobalt and other metal salts with resins, the art of which has been presented herein has not disclosed the exclusive use of an inorganic salt of nickel to increase adhesion properties between rubber and brass or brass-plated metallic reinforcement.